In conventional image forming apparatuses, various schemes are employed for improvement in printing speed. For example, in an image forming apparatus using an ink-jet method in its printing unit, a black (Bk) printhead has a length longer than that of a color printhead such that the black printhead can perform printing in a wider printing range, thereby the printing speed is increased.
However, in the above-described conventional ink-jet image forming apparatus, although the printing speed in monochrome image printing has been improved by a longer Bk head, the printing speed in color printing has not been especially changed. Accordingly, if a monochrome image is processed as a color image, the printing speed is not improved.
Further, in a case where color image duplication is performed by an image forming apparatus having an original reading unit, if a monochrome image including characters or the like is processed as a color image due to color shift upon reading, printing is performed by using color ink or the like, thus pure monochrome printing cannot be performed.